Femoral hip prosthesis and method of implantation

ABSTRACT

Implants and methods are presented for surgically repairing a hip joint with a proximal femoral prosthesis that comprises femoral head component and a femoral stem component. The femoral stem component comprising a neck portion, a flange portion, a transitional body region and an elongated stem. The femur is prepared for implantation of the femoral hip prosthesis by resecting the proximal femur and reaming a symmetric intramedullary cavity in the femur. The femoral hip prosthesis is then inserted the on the resected femur and in the intramedullary cavity. The femoral hip prosthesis elastically deforms when loaded during use to apply dynamic compressive loads and displacement to the calcar region of the resected proximal femur.

CROSS-REFERENCE TO RELATED APPLICATIONS

Not applicable.

BACKGROUND OF THE INVENTION

1. The Field of the Invention

The present invention relates to a femoral hip prosthesis for replacinga portion of proximal femoral bone during hip replacement and themethods of assembly and use thereof.

2. The Relevant Technology

Total hip arthroplasty using a metallic hip prosthesis has beensuccessfully performed since the early 1960's and is now a routineprocedure to address orthopedic diseases such as osteoarthritis,fracture, dislocations, rheumatic arthritis, and aseptic or avascularbone necrosis. During this procedure, the bone is prepared for theprosthesis by removing the damaged articulating end of the bone byresecting a portion of the bone including the femoral head. This exposesthe inside, of the metaphaseal region of the intramedullary canal in theproximal femur. The surgeon then drills or reams a cavity in the femurapproximately in line with the intramedullary canal. This cavity is usedto align other tools such as reamers, broaches and other bone tissueremoval instruments to create a roughly funnel shaped bone cavity thatis smaller in cross-section as it extends down from the bone resectionat the proximal end of the femur into the distal intramedullary canal.This funnel shaped cavity is typically also eccentric with more bonematerial removed from the medial calcar region of the proximal femurthan the region on the lateral side of the canal.

Oftentimes a grouting agent commonly referred to as bone cement is thenadded to the funnel shaped cavity. Once the prosthesis is inserted intothe cavity, this creates a bone cement mantle between the prosthesis andthe bone. Sometimes the shape of the cavity is prepared to closely matchthe shape of the external surface of the prosthesis, and the prosthesisis press fit into the cavity without the use of bone cement. Thesepress-fit prostheses typically have a textured bone-ingrowth surfacesplace strategically at specific locations on their surface to helpfacilitate lone-term bone tissue growth into the prosthesis. This boneingrowth into the porous structure on the implant creates a long lastingsecure bond between the prosthesis and the proximal femur.

Once the bone cavity is prepared, the prosthesis is placed into the bonecavity and is supported directly by internal bone tissue in the case ofa press fit implant or indirectly by the bone cement mantle in the caseof the cemented implant. Then, the prosthesis is aligned such that thearticulating end of the implant articulates with the opposite side ofthe natural joint in the case of a hemiarthoplasty, or articulates witha corresponding implant replacing the opposite side of the joint in thecase of a total joint arthroplasty.

Current designs of proximal femur hip prosthesis have eccentric,non-symmetric cone shaped central body portions. The current methods ofimplant fixation allow for transfer of axial loads to the proximal femurmainly through shear stresses at the eccentric funnel shapedbone-prosthesis interface. The effective transfer of load issignificantly dependent on the three-dimensional shape of funnel shapedcavity, the bone-prosthesis or bone-cement-prosthesis interface as wellas physiological loading of the proximal end. Partly because of theeccentrically shaped cross-section of the central body portion, thesecurrently available prostheses transmit radial expansion forces on theproximal femoral cavity as the implant is loaded in compression. Thefunnel shape of the cavity and the matching shape of the implant or bonecement result in circumferential hoop stresses and radial expansionstresses are distributed to the bone as the femoral component is axiallyloaded. This results in complex axial and shear stresses at thebone-implant interface. Consequently, the distribution of the loads thattransmit from the femoral head axially through the proximal femur isaltered after THA.

A potential cause of failure of currently used prosthesis is associatedwith the possible resorption of the bone surrounding the implant. Thebone resorption can be the result of an altered distribution of shearstresses on the remaining proximal femoral tissue. In time, the lack ofadequate stress transfer from the metal stem to the surrounding bone maycause a loss of bone density, resulting in the increased possibility ofbone failure or loosening of the bone-prosthesis interface. The gradualloss of bone support in the calcar region of the eccentric cavityincreases the bending load that must be borne by the prosthesis. Thisincrease in bending load on the prosthesis can lead to stress shieldingby the prosthesis resulting in prosthesis fatigue and potentially toeventual clinical failure.

SUMMARY

The present invention is directed to a femoral hip prosthesis thatsatisfies the need for anatomically distributing the dynamic compressiveloads on the hip joint to the proximal femoral bone. The femoral hipprosthesis is adapted for implantation against a resected surface on aproximal end of a femur, and also in an intramedullary cavity of thefemur. The femoral hip prosthesis comprises femoral head component and afemoral stem component. The femoral stem component comprises a neckportion, a flange portion, a transitional body portion, and an elongatedstem portion. The neck portion comprises a proximal male friction fitportion and a distal neck body. The flange portion is distal andadjacent to the neck portion and is attached to the distal neck body.The flange portion comprises an upper portion and a bottom surface. Thetransitional body region is adjacent to the bottom surface of the flangeportion and also extends from the distal neck body. The elongated stemportion extends distally from the transitional body region and isaligned with a longitudinal axis. The longitudinal axis is oriented atan acute angle relative to the bottom surface of the flange portion. Theelongated stem portion comprises a uniform envelope that may containrotation-restricting splines, a tapered portion or a transverse slot.The femoral hip prosthesis may also alternatively contain arotation-restricting boss that is attached to the bottom of the flangeportion. The femoral hip prosthesis also comprises a distal end tipportion on the distal end of the elongated stem portion.

DESCRIPTION OF THE DRAWINGS

Various embodiments of the present invention will now be discussed withreference to the appended drawings. It is appreciated that thesedrawings depict only typical embodiments of the invention and aretherefore not to be considered limiting of its scope.

FIG. 1 is a perspective view of the show from the anteriomedialdirection showing the femoral hip prosthesis including the before it isinserted into the resected proximal femur;

FIG. 2 is a perspective view shown form the anterioromedial directionshowing the femoral hip prostheses before it is inserted and across-sectional view of the resected proximal femur with theintramedullary cavity and the boss cavity prepared;

FIG. 3 is a perspective view shown from the posteriorolateral positionshowing the before it is inserted into the resected proximal femur;

FIG. 4 is an anterior side view of the femoral hip prosthesis shownoutside of the femur;

FIG. 4 a is an embodiment of a substantially circular cross-section ofthe elongated stem portion;

FIG. 4 b is an embodiment of a substantially square cross-section of theelongated stem portion;

FIG. 4 c is an embodiment of a substantially triangular cross-section ofthe elongated stem portion;

FIG. 4 d is an embodiment of a substantially hexagonal cross-section ofthe elongated stem portion;

FIG. 4 e is an embodiment of a substantially star shaped cross-sectionof the elongated stem portion;

FIG. 5 is an isometric view of the medial side of the femoral stemcomponent as it appears outside of the femur;

FIG. 6 is a cross-sectional view of the proximal femur from the anteriorside showing the positioning of the femoral stem component inside of theproximal femur;

FIG. 7 is a medial view of the femoral hip prosthesis inside of theproximal femur.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Depicted in FIGS. 1 through 7 are different embodiments of animplantable proximal femoral hip prosthesis 50 and methods ofimplantation therein. These embodiments of a hip joint replacementprocedure and the design of the femoral hip prosthesis 50 that is meantto restore the biomechanical function of the hip joint while maintaininga secure interface with the proximal femur 10 and help to preserveanatomical loading of the remaining bone that surrounds the femoral hipprosthesis 50 once it is implanted. This allows the loads on the hipjoint to be distributed optimally to the proximal femur 10.

The femoral hip prosthesis 50 comprises a femoral head component 700 anda femoral stem component 100. The femoral stem component 100 comprises aneck portion 150, a flange portion 200, a transitional body portion 300,an elongated stem portion 400, and a distal tip end 500. Thenon-eccentric symmetrical shape of the interface between the elongatedstem portion 400 of the femoral stem component 100 and a cavity 25 alongwith the contact at the interface between a proximal resection 20 andthe femoral stem component 100 helps to stabilize the femoral hipprosthesis 50 and transfer more anatomic loads from the prosthesis 50 tothe bone efficiently.

To prepare the patient for a proximal femoral hip prosthesis 50, thesurgeon first makes an incision or incisions near the hip joint, thenthe surgeon cuts though some of the tissue near the articulating joint,and retracts these tissues apart to visualize and access the diseasedbone structures that are to be replaced by the hip joint replacementprostheses. FIG. 1, is a simplified perspective view from theanteriomedial direction showing the proximal femur 10 and the femoralhip prosthesis 50, showing the femoral head component 700, the femoralstem component 100, and the proximal femur 10. For clarification, all ofthe other tissues necessary to provide function to the hip joint are notshown in this simplified view. The surgeon aligns bone tissue removaltools, such as drills, reamers, or broaches (not shown) with alignmentinstrumentation (not shown) to form a substantially non-eccentric,symmetric intramedullary cavity 25 in the cancellous bone 3 of theproximal femur 10 that is in longitudinal alignment with the shaft 45 ofthe proximal femur 10. The intramedullary cavity 25 is formed with across-sectional shape, such as a diameter 46 as shown in an embodimentof the intramedullary cavity 25 shown in FIG. 2. The diameter 46 is themeasurement of the diameter of the maximum circular periphery thatencompasses an envelope that the cross-sectional shape of theintramedullary cavity 25 comprises. The shape of the intramedullarycavity 25 can also be substantially non-eccentric, symmetric,non-circular shapes such as a square (not shown), star shape, (notshown), hexagon (not shown), or other parallelogram shape. Matchingnon-circular shapes of both the femoral stem component 100 and theintramedullary cavity 25 are potentially more efficient at restrictingtorsional movement between the femoral stem component 100 and theproximal femur 10 than circular cross-sectional shapes. Theintramedullary cavity 25 also is formed to a length 47 as also shown inFIG. 2. The cross-sectional shape, diameter 46 and length 47 of theintramedullary cavity 25 in the proximal femur 10 is dependent on themorphology, structure, and pathology of the patient anatomy and theanticipated biomechanical in vivo loads resulting from the use of thefemoral stem component 100.

The intramedullary cavity 25 may have a multiple diameters, or in thecase of non-circular cross-sectionally shaped cavities multiple sizes,to approximately match the shape of the femoral stem component 100. FIG.2, which illustrates a cross-sectional view of the proximal femur 10,shows that both a first diameter 46 and a second diameter 48 can form anintramedullary cavity 25 is shown in FIG. 2. The second diameter 48 inthe embodiment of FIG. 2 is smaller than the first diameter 46.Additionally, a third, forth, fifth, or more diameters (all not shown)can form the intramedullary cavity 25. For the purposes of clarity ofillustration, the cross-section of the intramedullary cavity on FIG. 1and FIG. 2 are circular, resulting in a substantially cylindricalopening. Correspondingly different size or shaped tissue removal tools(not shown) are used to prepare an intramedullary cavity 25 with morethan one size diameter 46. The surgeon may also find it advantageous toform different or alternating shapes of cross-sections in the symmetric,non-eccentric intramedullary cavity 25. For example, the cavity may befirst diameter circular, and then square, then a second diameter circle,then a star shape, then cone shaped, then finally spherically shaped atits deepest, most distal end.

After the basic intramedullary cavity 25 is formed, instrumentation (notshown) is used to align cutting guides for bone cutting instruments (notshown) to form a proximal resection 20 on the proximal femur 10. Theproximal resection 20 may have different surfaces such as a calcarresection surface 12 that is formed when the femoral calcar 11 istransverely cut through the proximal femur 10. The calcar resectionsurface 12 is cut at an acute angle 22 with respect to the longitudinalaxis 21 of the proximal femur 10. This acute angle is typically between10° and 80°. Although the proximal resection 20 may be simply onecontinuous transverse cut that passes from the medial to the lateralside of the proximal femur in the direction and plane defined by a theplane outlined by the dashed line 16 shown in FIG. 1. This alternativeresection 17 is formed by extending the calcar resection 12 from medialto lateral though the entire proximal femur 10.

More bone conserving cuts may also be formed in to the proximal femur 10as shown in FIG. 1. These cuts may include a formed concentric region 15that is larger in size but concentric to or aligned with theintramedullary cavity 25. These cuts may also include a transverseresection 13 that is cut relatively perpendicular to the intramedullarycavity 25. To simplify the surgical procedure, the resections shown inFIG. 1 can all be formed by a single reamer (not shown). This reamer hasa cutting surface formed in the shape of the combined profile of all ofthe resection cuts. It can be rotated or oscillated about thelongitudinal axis 21 of the intramedullary canal 25, until the desirebone tissue is removed. As shown in FIG. 2, the various cuts thattogether form the proximal resection 20 pass through portions of boththe relatively dense cortical bone 44 and the more porous cancellousbone 3. Thus, the cutting surfaces of the tissue removal tools aredesigned to cut both dense cortical bone 44 and less dense cancellousbone 3.

After the intramedullary cavity 25 and the proximal resection 20,including the calcar resection 12 and when applicable other bone tissueremoval cuts are formed, the femoral stem component 100 can be insertedto mate with the exposed bone surfaces. The femoral stem component 100comprises a proximal male friction fit portion 150, a distal neck body160, a flange portion 200, a transitional body portion 300, an elongatedstem portion 400, and a distal end tip portion 500. These portions willbe discussed in detail below.

The femoral stem component 100 has a proximal male friction fit portion150 on its most proximal end that is shaped to accept partiallyhemispherical femoral head component 700. One shape of the proximal malefriction fit portion 150 is a cylindrical taper shape with the smallerdiameter on the male friction fit portion proximal section 151, atapered male friction fit portion 152 distal to the male friction fitportion proximal section 151, and a larger diameter male friction fitportion taper maximum cross-section bottom end 153 on the distal end ofthe male friction fit portion 152. The proximal male friction fitportion 150 could also be a straight cylindrical shape without a taper,or a series of successively larger diameter cylindrical shapes.

A femoral head component 700 has a male cavity 720 that is dimensionedto fit over and mate with the friction fit portion 152 of the proximalmale friction fit portion 150 when the femoral head component 700 isassembled on the proximal male friction fit portion 150. The femoralhead prosthesis 700 has an external bearing surface portion 710 on itsexternal surface that is substantially on its proximal side whenimplanted. The external bearing surface portion 710 of the femoral headprosthesis 700 is substantially hemispherical shaped on a portion of itsload bearing external bearing surface. This hemispherical shape isdesigned to mate with either an artificial prosthetic acetabular cupsurface (not shown) as is the case for a total hip arthroplasty or anatural acetabular surface as is the case for a hip femoral hemiplasty.

The proximal male friction fit portion 150 has a male friction fitportion neck 154 that is distal to the male friction fit portion portion152 and adjacent to the male friction fit portion taper bottom end 153.This male friction fit portion neck 154 functions as an undercut relieffor the femoral head component 700 when assembled. Because the malefriction fit portion neck 154 is smaller in diameter than the malefriction fit portion portion 152, the femoral head component 700 can bepressed onto the proximal male friction fit portion 150 with the onlydirect contact between the two on the friction fit portion 152 of thefemoral stem component 100 and the male friction fit portion 720 of thefemoral head component 700.

The male friction fit portion neck 154 is proximal to and attacheddirectly to a more bulky distal neck body 160. The distal neck body 160is shaped to distribute the loads transmitted through the proximal malefriction fit portion 150 from the femoral head component 700 through aflange portion 200 and a transitional body portion 300. The shape of thedistal neck body 160 transitions from a simple symmetric shape similarto the cross-section of the male friction fit portion neck 154 to a morecomplex asymmetric shape that is similar to the combined shape of theflange portion 200 and the transitional body portion 300. In theembodiment shown in FIGS. 1 through FIG. 7, the cross-sectional shape ofthe distal neck body 160 at the proximal section is round because themale friction fit portion is a conical tapered and the male friction fitportion neck 154 is a cylindrical hourglass shape. However, the shape ofthe distal neck body 160 at the proximal section can be other shape tocorrespond with the shape of the proximal male friction fit portion 150.

The flange portion 200 has an upper portion 210 on its proximal sidethat contacts at least a part of the distal neck body 160. In theembodiments shown in FIGS. 1, 2, 3, 6 and 7 the flange portion is angledto match the at the same angle as the calcar resection 12 made by thesurgeon on the proximal femur 10. The angle and the size of the flangeportion 200 are dependent on the anatomy of the patient and themorphology of the calcar resection 12. The flange portion 200 has ananterior-posterior flange portion width 240 that is wide enough to coverat least a portion of the cortical bone tissue 44 that has beenresected. The cortical bone tissue 44 is more rigid than the cancellousbone tissue 3. In a healthy hip joint, the compressive loads aretransmitted through both the cancellous bone tissue and the corticalbone tissue 44 of the proximal femur 10. Because the cortical bonetissue 44 is more dense and rigid, and can sustain a higher load persquare unit area without fracture than the cancellous bone tissue 3,cortical bone tissue 44 is a more efficient distributor of compressiveloads than cancellous bone tissue 3. Thus, the flanged portion 200 isshaped to cover both the resected cancellous bone 3 and the resectedcortical bone 44 so that the compressive loads transmitted through theflange portion 200 are distributed as anatomically close as possible tohow they were distributed when the proximal femur 10 was healthy andintact.

The flanged portion 200 is less thick than it is wide. As shown in theembodiment of FIG. 4, the flange portion thickness 221 is between 0.5millimeters and 12 millimeters. The flange portion 200 is substantiallythick enough to transmit loads from the hip joint to the calcarresection surface 12 of both the cancellous bone tissue 3 and thecortical bone tissue 44. The flange portion 200 is also thin enough tolimit the about of bone that must be resected to form the calcarresection 12.

The transitional body region 300 is the portion of the femoral stemcomponent 100 that transitions from the distal neck body 160 and theflange portion 200 to the distal elongated stem portion 400. Thetransitional body region 300 is adjacent to both the distal neck body160 and the flange portion 200 on its proximal side and adjacent to theelongated stem portion 400 on its distal side. The transitional bodyportion 300 has a maximum height 310 that is the linear distancemeasured between a plane tangent to the bottom surface 220 of the flangeportion 200 and a plane through the most distal part of the transitionalbody portion 300. In FIG. 4, these two planes are shown as lines sincethis is a side view. In the embodiment of the transitional body portionshown in FIG. 4, the transitional body portion 300 has a curved fillet330 its medial side. Although this is shown as a round fillet in FIG. 4,the medial side of the transitional body portion 300 can be a chamferedfillet, a stepped fillet, or any other non-linear or linear shape thattransitions from the shape of the elongated stem portion 400 to theshape of portions of the distal neck body 160 or the flange portion 200.The maximum height 310 of the majority of the transitional body region300, when measured normal from the bottom surface 220 of the flangeportion 200 to any part of the elongated stem portion 400 is less thanthirteen millimeters or less. Both the physical structure of the femoralhip prosthesis 50, and the mechanical properties of the material fromwhich the prosthesis is fabricated, function together to determine thefunctional strength and elasticity of the femoral stem component 100.

Conventional orthopedic alloys such as cobalt chrome, titanium andstainless steel alloys and orthopedic composite materials have proven toprovide reasonable strength and rigidity to orthopedic implants and mayalso be used to fabricate the femoral stem component 100. However, whenconventional orthopedic alloys or composites are fabricated into theeccentric conical shape of a typical femoral stem component 100, theresulting implant is more rigid than the proximal femoral 10 that thefemoral stem component 100 is replacing. Flexibility of the stemcomponent 100 is necessary to allow the flex and compliance desired todynamically anatomically load the proximal femur 10 bone duringbiomechanical loading. The relatively small shape of the transitionalbody portion 300 allows for more flexion of the flange portion 200 whenthe proximal male friction fit portion 150 is loaded than is seen withthe bulkier conventional eccentric cone shaped femoral prosthesis. Theunique shape of the femoral stem component 100 allows for flexibility ofthe prosthesis even when fabricated from rigid orthopedic alloys such assuch as cobalt chrome, titanium and stainless steel alloys.

This dynamic flexibility within the transitional portion 300 is desiredsince it allows the flange portion 200 of the femoral stem component 100to transmit loads and displacements to the femoral calcar region 11 ofthe proximal femur 10. When bone is loaded and allowed to deform, apiezoelectric effect within the tissue simulate the bone cells intofurther production. This phenomenon, sometimes called Wolfs Law, coupledwith other physiologic and biochemical principles, helps to keep thebone surrounding the femoral hip prosthesis 50 healthy and vibrant. Thefemoral stem component 100 is designed to optimize the effects that aflexible, yet strong femoral hip prosthesis 50 will have on thesurrounding loaded bone tissue. As the hip joint is loaded duringclinical use, loads are transmitted through the male friction fitportion 154 and distal neck body 160 to the flange portion 200 and thetransitional body portion 300 to the stem. Since the transitional bodyportion 300 is relatively flexible and not as bulky and rigid as aconventional femoral hip prosthesis, the transitional body portion 300allows the femoral stem component 100 to flex and transmit thecompressive load to the bone in the calcar region 11 of the proximalfemur 10. These loads on the bone may allow the dynamization necessaryto keep the tissue surrounding the femoral stem component 100 healthyand help prevent bone resorption in the calcar region 11 of the proximalfemur 10.

Distal and adjacent to the transitional body portion 300 is theelongated stem portion 400. The elongated stem portion 400 comprisessome or all of the following portions and features; a tapered portion450, a splined section 470, and transverse slot 480. The elongated stemportion is encompassed within a cylindrically shaped envelope referredto as uniform envelope 410. The cross-sectional shape and the area ofthe uniform envelope 410 remains substantially uniform throughout thelongitudinal length of the elongated body. The uniform envelope 410 hasa circular uniform cross-sectional periphery 902 that is defined by themaximum cross-sectional peripheral diameter 905 of the elongated stemportion 400. The uniform envelope 410 is the same length as theelongated stem portion. The elongated stem portion is adjacent to thetransitional body portion 300 on its proximal end and adjacent to adistal tip portion 500 on its distal end.

As shown in FIG. 4, the elongated stem portion 400 is longitudinallyaligned with a longitudinal axis 425. When the femoral stem component100 is implanted in the proximal femur 10, the longitudinal axis 425 isapproximately in alignment with the longitudinal axis 21 of theintramedullary cavity 25. All the possible features or portions of theelongated stem portion 400, including the tapered portion 450, the spinesection 470, and the transverse slot 480 have cross-sectionsperpendicular to the longitudinal axis 425 and are contained within amaximum diameter 905 of a cross-sectional periphery 902 that defines thecross-section of the uniform envelope 410. Representative shapes ofcross-sectional areas viewed from a cross-sectional view cut plane 900are shown in FIG. 4 a through FIG. 4 e. Included in these figures arethe cross-sectional periphery 902 and the maximum diameter 905 of thecross-sectional periphery 902.

Material may be removed from the elongated stem portion 400 to createdfeatures such as taper portions 450, splines 460 or the transverse slots480. However, the basic substantial shape of the external periphery ofthe cross-section of the elongated stem portion 400 remains uniform andcircular. Thus, the elongated stem portion and the uniform envelope 410are both substantially symmetric and non-eccentric. The embodiment ofthe elongated stem portion 400 shown in FIG. 4 is substantiallycylindrical in shape 910. The cross-section of this cylindrically shapedelongated stem portion is shown in FIG. 4 a. However, for otherembodiments of the femoral stem component 100, the cross-sectional shapeof the elongated stem portion 400 can be also non-circular shapes suchas substantially square shape 920, as shown in FIG. 4 b; a substantiallytriangle shape 930, as shown in FIG. 4 c; a substantially hexagonalshape 940, as shown in FIG. 4 c, a substantially star shape 950 as shownin FIG. 4 e, or any other substantially non-eccentric, symmetric shapesuch as a tube (not shown) that can functionally form the cross-sectionof the elongated stem portion 400.

In the embodiments shown, the longitudinal axis 425 of the elongatedstem portion 400 is a substantially straight axis throughout the lengthof the elongated stem portion 400. However, to better match the anatomyof the proximal femur 10, the longitudinal axis 425 can also be curved.The curve may be in the anterior-posterior plane, the medial-lateralplane or a compound curve that is seen in both the anterior-posteriorplane and the medial-lateral plane. A flexible reamer (not shown) couldbe used to form the curved intramedullary cavity before the prosthesis10 with a curved longitudinal axis 425 is implanted.

The elongated stem portion 400 may include a tapered portion 450 alongits length. This is shown in FIG. 2. This tapered portion 450 may alsoinclude splines 460 or transverse slots 480 cut into it. Thecross-sectional area of the tapered portion 450 in the embodiments showndecreases linearly along the longitudinal length of the tapered portion450 as the tapered portion 450 transitions down the length of theelongated stem portion 400 from proximal to distal. The direction of thetapered portion 450 may also be in the opposite direction. The taperedarea in the elongated portion 400 allows for greater flexibility inbending along the tapered portion 450 due to the reduced cross-sectionalarea and reduced cross-sectional bending moment of inertia. The taperedportion 450 also allows for an interference tapered wedge fit betweenthe elongated stem portion 400 and the intramedullary cavity 25 in theproximal femur 10 when the cross-sectional size of the intramedullarycanal 25 is less than the maximum diameter 905 of the periphery 902.

Features such the splines 460 are cut into the elongated stem portion400 for various structural and functional reasons such as to provideadditional torsional resistance to the femoral stem component 100. Inthe embodiments shown, the splines 460 are evenly spaced around theperiphery 902 of the distal elongated stem portion 400. The splines 460are cut longitudinal around the periphery 902 of the elongated stemportion 400. This allows the splines 460 to resist axial rotationbetween the femoral stem component 100 and the intramedullary cavity 25.The splines 460 may also provide additional structural flexibility tothe distal end of the femoral stem component 100.

At the distal end of the femoral stem component 100, an optionallongitudinal transverse slot 480 may be cut transversely into theelongated stem portion 400 to provide additional flexibility andpotentially additional torsional resistance to the femoral stemcomponent 100. The embodiment of the slot 480 that is shown issubstantially uniform in cross-sectional and in shape though its length.The cross-sectional shape of the slot 480 may also be non-uniform. Thecross-sections shape of the slot 480 may also change. For example thesides of the slot 481 may change from parallel planar surfaces tonon-parallel or non-planar surfaces as the slot transitions from distalto proximal. The slot 480 also has a fillet 485 that takes the form of arounded radius shape at its most proximal end. The shape of this fillet485 may be other shapes that allow a relatively smooth transition fromthe slot 480 to the non-slotted cross-section. For example the slot 480may be keyhole shaped.

Adjacent and distal to the elongated stem portion is the distal end tipportion 500. The distal end tip portion 500 has a lead-in section 510that reduces in cross-sectional area from proximal to distal. Thelead-in section may be tapered as in the embodiment of FIG. 3, orspherical as in the embodiment of FIG. 4, or any shape that issuccessively smaller in cross-sectional area from proximal to distal.The distal end tip portion 500 helps to guide the femoral stem component100 into the intramedullary canal 25. The relatively smooth shape of thedistal end tip portion 500 also functions to reduce the stress on theproximal femoral bone associated with discontinuity of terminating arigid prosthesis in the intramedullary canal 25.

The load distribution on the proximal femur 10 of an intact hip jointcan be essentially resolved into an axial component, a bending moment inthe medial and lateral direction, a bending moment in the anteriorposterior direction, and a torsional moment with a rotational axisapproximately in line with the longitudinal axis 21 of the proximalfemur. The distribution of the magnitude and direction of these forcecomponents depend upon complex combinations of biomechanical factorssuch as leg stance, patient weight distribution, and patient gait. Thefemoral stem component 100 is designed to translate these forces toanatomic loads on the proximal femur 10. As described above, the flangeportion 200 helps to translate the compressive loads to the cancellousbone 3 and cortical bone 4 in the calcar region 11. The elongated stemportion 400 helps to transmit the bending and torsional moments to theintramedullary canal 25. In addition, a rotation-restricting boss 600helps to transmit some of the torsional moments to the bone in thecalcar region 11 of the proximal femur 10. As shown in FIG. 6, the bossperiphery 630 of the rotation-restricting boss 600 interfaces with thebone surrounding the boss cavity 14. This structural interferencebetween the femoral stem component 100 and the proximal femur 10 helpsto restrict rotation, caused by the above-described resultant torsionalmoment with approximately in line with the longitudinal axis 21 of theproximal femur 10, between the femoral stem component 100 and theintramedullary cavity 25.

The size and location of the rotation-restricting boss 600 are factorsthat affect the amount that the rotation-restricting boss 600 restrictsrotational movement of the femoral stem component 100. Due to greaterresistance from a rotation-restricting boss with a larger resultantmoment arm, the further that the rotation-restricting boss 600 islocated from the longitudinal axis 425 of the elongated stem portion400, the more effective it is in transmitting rotational loads andrestricting rotational movement of the femoral stem component 100 to theproximal femur 10. Also, the larger the cross-sectional area of therotation-restricting boss 600, the more effective it is in distributingtorsion and restricting rotational movement of the femoral stemcomponent 100.

The embodiments of the rotation-restricting boss 600 that are shown byexample are circular in cross-section resulting in a cylindrical shapedboss. However, other cross-sectional shapes such as square, rectangular,triangular or diamond shapes may be more practical to machine or may bebetter at distributing torsional loads from the femoral hip prosthesis50 to the proximal femur 10 than the shown cylindrically shapedrotation-restricting boss 600. The optimized shape of therotation-restricting boss 600 may be more fin shaped than cylindricalshaped or longer than it is wide. This shape is partially dependent onthe mechanical characteristics of the bone tissue where therotation-restricing boss 600 is inserted.

The rotation restricting boss 600 has an axis of protrusion 620 withorigin 621 substantially on a plane tangent or coincident with thebottom surface 220 of the flange portion 200. The boss axis ofprotrusion origin 621 and the stem portion longitudinal axis 425 arespaced apart by a length this is more than the maximum length 905 of thecross-section of the periphery 902 of the uniform envelope 410 of theelongated stem portion 400.

The rotation-restricting boss 600 in the embodiment of the prosthesis 10shown in FIG. 3 has an axis of protrusion that is substantially normalto the bottom surface 220 of the flange portion 200. Therotation-restricting boss in the embodiment of the prosthesis 10 shownin FIG. 4, FIG. 5 and FIG. 6 has an axis of protrusion 620 that issubstantially parallel with the longitudinal axis 425 of the elongatedstem portion 400. As shown in cross-section of the proximal femurillustrated in FIG. 6, the corresponding boss cavity 14 is in line withthe axis of protrusion 620.

As shown in FIG. 6 and FIG. 7, after the femoral stem component 100 isimplanted in the proximal femur 10, the flange portion 200 is pressedagainst a resected surface 20 on the proximal femur 10, and theelongated stem portion 400 is pressed in the intramedullary cavity 25aligned with the long axis of the proximal femur 10. As the femoral head700 is loaded by the hip joint, a substantial component of the axialcompressive force is transmitted to the cancellous 3 and cortical 4 bonein the calar region 11 from the flange portion 200. The principletorsional loads are transmitted to the splines 460, slot 480 androtational-restricting boss 600 and the principle bending loads aretransmitted to the intramedullary canal 25 through the elongated stemportion 400. Collectively, these feature and portions of the femoral hipprosthesis 10 contribute to distribute anatomical loads from hip jointto the remaining proximal femoral bone tissue.

While the present invention has been disclosed in its preferred form,the specific embodiments thereof as disclosed and illustrated herein arenot to be considered in a limiting sense, as numerous variations arepossible. The invention may be embodied in other specific forms withoutdeparting from its spirit or essential characteristics. The describedembodiments are to be considered in all respects only as illustrativeand not restrictive. No single feature, function, element or property ofthe disclosed embodiments is essential. The scope of the invention is,therefore, indicated by the appended claims rather than by the foregoingdescription. The following claims define certain combinations andsubcombinations that are regarded as novel and non-obvious. Othercombinations and subcombinations of features, functions, elements and/orproperties may be claimed through amendment of the present claims orpresentation of new claims in this or related applications. Such claims,whether they are broader, narrower or equal in scope to the originalclaims, are also regarded as included within the subject matter ofapplicant's invention. All changes that come within the meaning andrange of equivalency of the claims are to be embraced within theirscope.

1. A prosthesis adapted for implantation against a resected surface on aproximal end of a femur and inside of an intramedullary cavity of thefemur, the prosthesis comprising: a femoral head component comprising anexternal bearing surface; and a femoral stem component comprising: aneck portion comprising a proximal portion, engagable with the femoralhead component, and a distal neck body; a flange portion distal andadjacent to the neck portion, the flange portion comprising a bottomsurface; a transitional body region adjacent to the bottom surface ofthe flange portion and extending from the distal neck body; and anelongated stem portion extending distally from the transitional bodyregion and having a longitudinal axis oriented at an acute angle fromthe bottom surface of the flange portion; wherein the transitional bodyregion is shaped to flex such that, during a normal gait cycle, thebottom surface exerts a significant compressive load on the resectedsurface of the femur.
 2. A prosthesis as is claim 1, wherein theelongated stem portion comprises a uniform envelope with a substantiallyconstant cross-sectional peripheral shape and size.
 3. A prosthesis asin claim 1, wherein the elongated stem portion comprises a proximalsection having a cross sectional shape that is substantially consistentalong a longitudinal length of the proximal section, wherein a minimumdisplacement between the bottom surface of the flange and the proximalsection, measured normal to the bottom surface, is less than thirteenmillimeters.
 4. A prosthesis as in claim 1, further comprising arotation-restricting boss, extending from the bottom of the flangeportion.
 5. A prosthesis as in claim 2, further comprising arotation-restricting boss, extending from the bottom of the flangeportion.
 6. A prosthesis as in claim 5, wherein the rotation restrictingboss has an axis of protrusion with a boss axis origin near the bottomsurface of the flange, the elongated stem also has a stem axis originnear the bottom of the flange, the boss axis origin and the stem axisorigin are spaced apart by a length more than the maximum cross-sectionof the elongated stem portion.
 7. A prosthesis as in claim 6, whereinthe axis of protrusion and the longitudinal axis are substantiallyparallel.
 8. A prosthesis as in claim 6, wherein the axis of protrusionand the longitudinal axis are not substantially parallel.
 9. Aprosthesis as in claim 6, wherein the axis of protrusion is normal tothe bottom surface of the flange portion.
 10. A prosthesis as in claim1, wherein the elongated stem portion has a distal section with multiplelongitudinal flutes, wherein the longitudinal flutes are alignedapproximately parallel to the longitudinal axis.
 11. A prosthesis as inclaim 1, wherein the neck portion is aligned at an obtuse angle withrespect to the bottom surface of the flange portion.
 12. A prosthesis asin claim 11, wherein the obtuse angle is between 100° and 170°.
 13. Aprosthesis as in claim 1, wherein the neck portion has a first end and asecond end, wherein the first end is connected to the flange portion andextends proximally therefrom and the second end is shaped to press-fitinto the femoral head component.
 14. A prosthesis as in claim 13,wherein at least a portion of the outer surface of the femoral headcomponent is hemispherical.
 15. A prosthesis as in clam 1, wherein theacute angle ranges from 15° to 80°.
 16. A prosthesis as in claim 2,wherein the uniform envelope has a maximum cross-section area measuredon a plane perpendicular to the longitudinal axis.
 17. A prosthesis asin claim 1, wherein the elongated stem portion has a length of at leastone hundred millimeters as measured along the length of its longitudinalaxis.
 18. A prosthesis as in claim 1, wherein the elongated stem portioncomprises a tapered portion.
 19. A prosthesis as in claim 1, wherein theelongated stem portion comprises a proximal section having a crosssectional shape that is substantially consistent along a longitudinallength of the proximal section, wherein a minimum displacement betweenthe bottom surface of the flange and the proximal section, measurednormal to the bottom surface, is less than a maximum cross sectionalwidth of the elongated stem portion, measured perpendicular to thelongitudinal axis.
 20. A prosthesis as in claim 1, wherein thetransitional body region is shaped to provide a lateral offset betweenan axis of the neck portion and the longitudinal axis of the elongatedstem portion.